In a liquid crystal display device, a liquid crystal is a substance that does not itself radiate light. Instead, the liquid crystal relies on light received from a light source, thereby displaying images and data. In the case of a typical liquid crystal display device, a backlight module powered by electricity supplies the needed light.
FIG. 12 (Prior art) represents a first typical backlight module 100. The backlight module 100 includes a light source 10, a light guide device 12, and a plurality of complementary optical elements 11, 13, and 14. The light guide device 12 includes an incident surface 122 facing the light source 10, an emitting surface 124 located at a top thereof, and a reflecting surface 126 opposite to the emitting surface 124. The complementary optical elements include a reflective sheet 11, a diffusion sheet 13, and a brightness enhancement sheet 14. The reflective sheet 11 is positioned under the light guide device 12 and is configured for reflecting light back into the light guide device 12. The diffusion sheet 13 is located above the light guide device 12 and is configured for uniformly diffusing the emitted light, thereby avoiding the occurrence of light spots on the emitting surface 124 of the light guide device 12. The brightness enhancement sheet 14 is positioned above the diffusion sheet 13 and is configured for collimating the emitted light, thereby improving the brightness of light illumination. However, due to employing these complementary optical elements, the backlight module is unduly complicated and therefore is costly to manufacture.
The light guide device 12 converts the linear light source 10 into a surface light source. However, the light guide device 12 by its very nature cannot control the emergence direction of the light emitted therefrom. FIG. 12 shows a typical light path 15 associated with the backlight module 100. A light beam 101 emitted from the light source 10 enters into the light guide device 12 through the incident surface 122 thereof. The light beam 101 is reflected at the reflecting surface 126 and then exits from the emitting surface 124. However, the light beam 101 emitting from such a light guide device 12 is generally not perpendicular to the emitting surface 124. Therefore, the complementary optical elements, such as the diffusion sheet 13 and the brightness enhancement sheet 14, have to be employed so as to direct the light beam to exit from the light guide device 12 in a direction that is perpendicular to the emitting surface 124.
A second conventional backlight module 30 which can restrain the light emitting angle within a certain range is shown in FIG. 13. The backlight module 30 includes a light source 31, a light guide device 32, and an array of prisms 33. The light guide device 32 has an incident surface 322, an emitting surface 324, and a bottom surface 326. The incident surface 322 is disposed adjacent the light source 31, the emitting surface 324 adjoins the incident surface 322, and the bottom surface 326 is opposite to the emitting surface 324.
The array of prisms 33 is formed on the emitting surface 324. Each prism 33 has an index of refraction higher than that of the light guide device 32. Each prism 33 has first, second, third, and fourth sides 334, 335, 336, 337. The first side 334 is brought into optical contact with the light guide device 32 at emitting surface 324. The fourth side 337 is opposite to the first side 334. The second and third sides 335, 336 adjoin the first and fourth sides 334, 337, respectively. Either of the second and third edges 335, 336 and the first side 334 cooperatively form an acute angle.
Upon being totally internally reflected, the light is directed to exit from the light guide device 32 and enters into the prisms 33 through the first side 334 thereof. The light is then reflected by the third side 336 of the prisms 33 and exits the prisms 33 through the fourth side 337 thereof as a spatially directed light source. However, even though light is successfully redirected by such a module 30, it is difficult to mass produce the light guide device 32 by way of a conventional mold injection method.
A third conventional light guide device 40 is shown in FIG. 14. The light guide device 40 includes a body 401 configured for guiding light transmitted from a tubular light source 41. The body 401 has a plurality of projections 422 arrayed on an emitting surface 42. The projections 422 are parallel to each other and extend parallel to a longitudinal direction of the tubular light source 41. A height of each of the projections 422 progressively increases from a central region thereof to the two opposite ends thereof.
However, the light guide device 40 has its drawbacks. The projections 422 of the light guide device 40 are unduly complicated in structure and therefore are costly to manufacture. In particular, the light guide device 40 cannot be easily/readily mass-produced by way of injection molding. Furthermore, the light guide device 40 cannot control the emitting light beams in a manner so as ensure that the beams exit perpendicularly from the emitting surface 42 of the light guide device 40. Therefore, the light guide device 40 generally still needs to employ additional optical correcting elements in order to redirect the light beams coming from the emitting surface 42 of the light guide device 40.
A fourth conventional backlight module 50 which can control the light emitting angle is shown in FIG. 15 and FIG. 16. The backlight module 50 includes a light source 51 and a wedge shaped light guide plate 52. The light guide plate 52 includes a multi-layer structure 53 formed on a bottom surface thereof. The multi-layer structure 53 includes a plurality of layers with an outmost layer having a plurality of projections 54. The layers each have different respective indices of refraction. However, as such, the multi-layer structure of the light guide plate 52 is unduly complicated in structure and therefore is costly to manufacture.
What is needed, therefore, is a light guide device which can control a plurality of emitting light beams to emit uniformly and substantially perpendicular to an emitting surface thereof and which can realize the function of a conventional backlight module without the aid of additional optical elements.